text: Ring expansion and contraction reactions are common in organic synthesis, and are frequently encountered in pericyclic reactions. Ring expansions and contractions can involve the insertion of a functional group such as the case with Baeyer–Villiger oxidation of cyclic ketones, rearrangements of cyclic carbocycles as seen in intramolecular Diels-Alder reactions, or collapse or rearrangement of bicyclic compounds as several examples.